[126] 500SEL fuel leak after long sitting

[psinthos]

My elderly s-classes are giving me a hard time.

I described injection/ignition-related problems resulting in idling irregularities in my 140 recently in another thread and, having experienced them again yesterday, decided to put it in for a diagnostic service. With the 140 in for a service, I decided to let the 126 stretch its legs a little, especially since it has been sitting in the garage for around four weeks.
Having fired the car up I took it out of its berth and stepped out to take assess if everything is in order (no weird noises, leaks etc.). I was disappointed to find a problem which had occurred last year return. To give you some background on the matter - last year the car had a couple of periods of non-usage lasting 1,5-2 months. Following sitting in the garage for so long, after the fuel pump was turned on, a large leak appeared from around the fuel pump/filter assembly, only to disappear after a couple of minutes of the pump's operation (the fuel pump on the 126 operates continuously, unlike that on 123s or 140s, for instance), so it can be left operating without actually starting the engine. Thereafter, during more regular operation, the leak would never return.

Last year I took the car in for a service to get this matter sorted out. The garage kept it for around a fortnight, operating the pump every few days and observing if there was a leak. There were none. Under such circumstances, all seals/washers in the fuel pump and filter assembly were replaced. The leak had never returned, even during longer periods of non-usage (e.g. in the winter) - until last night.

Last night, having ascertained that the leak is substantial (to put things lightly - see photo below), I shut down the engine. I then let the pump operate a few times for around 45 seconds at a time. As last year, the leak went away. Once the petrol that had leaked out evaporated, I fired the 126 up again and continued to observe the fuel pump assembly. It remained as dry as a dead dingo's donger, as the Aussies would put it.
Having determined that the risk of the 126 going up in smoke was pretty much eliminated, I proceeded to take it out for a test drive. After a good 30km of driving at various speeds and checking for leaks every few kilometres, I returned home.

Further analysis of the fuel pump assembly itself, and also of the photos I had been taking (with my cellphone) lead me to discover that the fuel pump and fuel filter, as well as the metal fuel lines leading to them had remained dry throughout the events described above. The leak appeared to have been originating from either the fuel pressure accumulator or the rubber hoses around it (judging especially by the stains left on the plastic cover and the accumulator itself - see pics).

This morning everything was fine and completely dry. I operated the pump first and then after a couple of minutes fired the car up. No leaks to be seen. I took the car on my daily commute to the office this morning and, again, it did not leak (which, of course, is great news).
The car is going in for a service and close-up examination of the fuel pressure accumulator and rubber hoses in the fuel pump assembly - it is likely that these will be replaced just in case. I am, however, quite baffled by the phenomenon of the huge leak appearing after a longer period of non-usage and then quickly disappearing - as though some of the connections were drying up and leaking and then re-sealing themselves once fuel flows through them and pressure is built up again. Has anyone seen anything like this every happening before? Is it possible that the accumulator itself actually leaks?

Some pictures of the leak are below - I apologise for their quality (or, rather, lack thereof), but they were taken ad hoc with a cellphone.

The leak almost immediately after operating the pump following four weeks of non-usage:


The situation after the leak disappearing and conducting a test drive:



A close-up of the fuel pressure accumulator, with what appear to be leak-stains on the accumulator body:

[psinthos]

As I had presumed, the culprit of the leak turned out to be the fuel pressure accummulator.

A new one was fitted (cost of the part itself - PLN 570, the equivalent of around USD 250).

At the same time the airconditioning was attended to, since its efficiency appeared to have deteriorated recently. It turned out the sender unit for the fan was faulty. A replacement was fitted.